I need a suitable firewall.

Hello guys I need some professional advice.
I have Servers in my co-lo data centre and at the moment they are live to the public internet. I currently have 4 servers running this will increase over time to about 12 servers. One of my test servers on Linux was hacked into the other day and they started to use that server to hack other PC/server. Not surprising as it has most simple password in the world and had no firewall at all.
I knew I needed a firewall and it was on its way. My programmer who was going to take this firewall task on has fallen ill and I have the task of finding a suitable firewall for my server in the data centre.
I had pre ordered a dedicated server for this task, (2.8 xeon. 80gb IDE 512ram). all I can remember of what my programmer said is that he was going to run a firewall on Linux and it was a free firewall. I have looked on the internet and I have found two good free firewall IPcop and smoothwall.
Now. These firewall I have trust in when it’s put in to an office environment, but I have very expensive software on the servers and important data to keep safe.
I need to have very fast packer switching firewall as one of my server is used for Voip.

My question is. Is IPcop going to be good enough for this or should I fork out and pay for a very good firewall software?
Can anyone recommend me a firewall for Linux which will keep my servers safe from hackers and does good data transfer?

I have a small webserver on a sarge box who is directly online with the Internet without any protection at all except a small iptables script. Works great and very easy to install. Just enter the ports you want to have open and start.
You can read about the KISS here http://www.geocities.com/steve93138/